Add support for hardcoded indexes in gld

[johnklipi]

I think there should be an option in PolyMod config to enable hardcoded custom indexes. If it is enabled, and, for example, a custom improvement doesnt have idx -1 but instead something like 67, it should create enum cache with idx 67. (but only in case if idx doest exist already, if it does it should throw. Also, as I said it should be working only with the option in config enabled. (I am doing one of my mods in which i would much prefer having hardcoded indexes)